I was placing horses in creative for some testing, when using the spawn egg I got a donkey (I wanted a horse). So I proceeded to try and kill it. When I hit the Baby donkey with a diamond sword it somehow went through the fence with its legs up in the air.

*What I expected to happen was that the donkey would stay in the pen and I would be able to kill it.

*What actually happened was that the donkey went through the fence and ran away

Steps to Reproduce:
1. Make a 4x3 Wooden fence cage at the Y cords of 8.000
2. get a baby donkey in it
3. when its legs are thrown up in the air hit it with a diamond sword.
4. Should go through the fence and not die
( I tried with horses, when the baby horses legs are up he can go through also when they are hurt and running around in panic they can walk right through the fence)

I have also attached a picture of the environment, not exactly right because its a baby Horse, not a donkey but I hope the point is made